Added a default value for XQUEUE_RABBIT_HOSTNAME

Was causing a flood of error logs in /mnt/logs/edx.log

Oct 11 15:48:21 hostname [queue.management.commands.run_consumer][env:sandbox] INFO [hostname  1630] [run_consumer.py:69] -  [*] Starting queue consumers...
Oct 11 15:48:21 hostname [pika.callback][env:sandbox] DEBUG [hostname  1630] [callback.py:161] - Added: {'callback': <bound method Worker.on_connected of Worker (u'http://localhost:18050', u'edX-DemoX')>, 'only': None, 'one_shot': False, 'arguments': None}
Oct 11 15:48:21 hostname [pika.callback][env:sandbox] DEBUG [hostname  1630] [callback.py:161] - Added: {'callback': <bound method SelectConnection._on_connection_start of <pika.adapters.select_connection.SelectConnection object at 0x2383990>>, 'only': None, 'one_shot': True, 'arguments': None, 'calls': 1}
Oct 11 15:48:21 hostname [pika.connection][env:sandbox] DEBUG [hostname  1630] [connection.py:810] - Attempting connection
Oct 11 15:48:21 hostname [pika.adapters.base_connection][env:sandbox] DEBUG [hostname  1630] [base_connection.py:98] - Connecting the adapter to the remote host
Oct 11 15:48:21 hostname [pika.adapters.base_connection][env:sandbox] DEBUG [hostname  1630] [base_connection.py:153] - Creating the socket
Oct 11 15:48:21 hostname [pika.adapters.base_connection][env:sandbox] INFO [hostname  1630] [base_connection.py:163] - Connecting fd 5 to $XQUEUE_RABBIT_HOSTNAME:5672
Oct 11 15:48:21 hostname [pika.adapters.base_connection][env:sandbox] ERROR [hostname  1630] [base_connection.py:109] - socket error: Temporary failure in name resolution
Oct 11 15:48:21 hostname [pika.adapters.base_connection][env:sandbox] ERROR [hostname  1630] [base_connection.py:118] - Could not connect: Temporary failure in name resolution
Update main.yml
Adding Philippe Chiu in AUTHORS since the CA has been signed.
Switching LOGLEVEL from DEBUG to INFO

Too much logs in DEBUG..
Merge pull request #331 from pchiu33/bugfix/pchiu33/missing_XQUEUE_RABBIT_HOSTNAME

Bugfix/pchiu33/missing xqueue rabbit hostname